A majority of the players tested at the Under-17 World Cup in Mexico had traces of clenbuterol in their bodies because they ate contaminated meat, FIFA said on Monday.
Tests in Germany after the tournament revealed players from 19 of the 24 teams had adverse findings of the banned anabolic agent in 109 of 208 urine samples.
FIFA medical officer Jiri Dvorak called the results ''highly surprising'' but insisted that teenage footballers were not cheating.
''It is not a problem of doping, but a problem of public health,'' Dvorak told reporters, adding that none of the players was harmed or put in any danger.
FIFA and the World Anti-Doping Agency declined to prosecute any cases because the weight of evidence pointed to contamination.
Mexican authorities have acknowledged the country has issues with feeding banned steroids to livestock.
''It's extremely serious for WADA,'' Olivier Niggli, the anti-doping watchdog's legal director, said in a conference call. ''Now it's known it's an issue, warnings are going to be sent.''
With Mexico currently hosting the two-week Pan American Games in Guadalajara, athletes have been advised to eat only in designated cafeterias.
Dvorak said FIFA would not identify the players or teams involved in the positive tests at the tournament played in seven cities in June and July
Mexico's players, however, who won the tournament were pronounced as all testing clean because they were switched to a fish and vegetables diet before the competition, Dvorak said.
Team management took those precautions when five of Mexico's senior squad were suspended for testing positive for clenbuterol at a training camp for the Gold Cup.
WADA wanted those players banned by the Court of Arbitration for Sport, but dropped their appeals last week when new FIFA evidence suggested the Mexico Football Federation was right to exonerate them.
''Since day one we knew the players were innocent and we are happy with this result,'' Justino Compean, the Mexican Football Federation president, said Monday.
The Mexican cases have confused the legal certainty of prosecuting athletes who test positive for clenbuterol - notably three-time Tour de France winner Alberto Contador.
Contador had minute traces of clenbuterol in samples taken during his 2010 Tour victory, and will face WADA and the International Cycling Union in a landmark case at CAS next month with his title and integrity at stake.
The Spanish rider claims contaminated beef that was bought from a butcher in his native country was responsible for his failed doping tests.
Niggli declined to draw comparisons between football and cycling cases, or Mexico and elsewhere.
''I don't think we can generalize from what is happening in one specific country,'' Niggli said, but acknowledged that China also had issues with livestock being fed banned drugs to improve the quality of meat.
''It is an illusion to say Mexico is the only country, but definitely there is a big difference depending where you are coming from,'' the WADA lawyer said.
Dvorak said FIFA had ''no indications'' of contamination problems in Spain or other countries.
Contador and the footballers in Mexico all had their urine tested by the WADA-accredited lab in Cologne, Germany.
While Contador's sample had 50 picograms of clenbuterol per milliliter, the Under-17 World Cup tests showed a ''majority'' tested between 50 and 300.
Dvorak said FIFA was alerted to potential contamination when three players gave samples of 300-1,300 picograms in the opening days of the three-week tournament, in tests conducted by the WADA lab in Los Angeles.
Mexican health official Mikel Arriola said authorities have begun a program of arresting farmers and shutting down slaughterhouses.
''We are going to continue these inspections in order to avoid poisoning the general population and doping (athletes),'' Arriola said.
